Magnetic resonance devices are devices for obtaining (measuring) physicochemical information of an object by irradiating a radio frequency magnetic field of a specific frequency (RF pulse) on the object placed in a static magnetic field to induce a nuclear magnetic resonance phenomenon. Among the measurements using magnetic resonance devices, magnetic resonance imaging (MRI) is currently widely used, in which physical parameters varying depending on types of biological tissues such as proton density and difference of relaxation time are imaged. This technique also includes imaging in which signal intensities are modulated with physical parameters.
The measurements using magnetic resonance devices also include a technique called spectroscopic imaging (SI), in which not only the proton density is measured, but also density distribution is measured for each of molecular species including hydrogen atom (1H), phosphorus atom (31P), fluorine atom (19F), carbon atom (13C), oxygen atom (17O), and so forth. This is a technique of separating magnetic resonance signals into those of each molecule on the basis of differences of magnetic resonance frequencies (chemical shifts) induced by differences of chemical bonds in the molecules, and imaging density, relaxation time or the like of each molecular species. In SI, in order to obtain the multi-dimensional information, i.e., chemical shifts and spatial information, it is necessary to scan a multi-dimensional measurement space, and thus the measurement time is prolonged. As a means for solving this problem, a method of scanning such a multi-dimensional measurement space in a short time by using an oscillating gradient has been proposed (refer to, for example, Non-patent document 1). Moreover, in the scanning with an oscillating gradient, the maximum value, the maximum slew rate, and so forth of the gradient magnetic field are restricted, and therefore a technique called interleaving in time domain has been proposed, in which the measurement is repeated with shifting starting time of the addition of the oscillating gradient to fill the k-space (refer to, for example, Non-patent documents 2 and 3 and Patent document 1).
One of the physical parameters attracting attention in recent years in view of the high clinical significance thereof is the diffusion coefficient, which represents intensity of molecular diffusion. The diffusion coefficient measured in MRI strongly reflects properties of tissues or cells, and differs from diffusion coefficient of simple molecular diffusion, and therefore it is called apparent diffusion coefficient (ADC). Pulse sequences currently widely used for measuring this ADC are those based on the pulse sequence of Stejskal-Tanner (refer to, for example, Non-patent document 4). In this measurement technique, after nuclear spins are excited with a radio frequency magnetic field, two or more self-compensating gradient magnetic fields are added to obtain signals. The term “self-compensating” used here means that influences of rotating phases of nuclear spins are offset unless molecules are moving. In a typical example, two gradient magnetic fields having the same shape, duration and amplitude, but added with positive and negative sign, respectively, are given.
When there is molecular diffusion, even if self-compensating gradient magnetic fields are added, the influence of rotating the phase cannot be completely offset, and signal intensity is attenuated at a rate corresponding to the added durations, intervals, and amplitudes of the gradient magnetic fields. A technique using the above attenuation for obtaining a diffusion weighted image is called diffusion weighted imaging (DWI), in which intensities of signals representing characteristics of tissues are attenuated according to the intensity of diffusion. The measurement is further performed a plurality of times with varying durations, intervals and amplitudes of the gradient magnetic fields, ADCs are calculated from the obtained attenuation factors of the signal intensity and imaged to obtain an ADC map. The diffusion weighted imaging, measurement of ADCs, and acquisition of ADC map are collectively called diffusion measurement.
One of the objects of the diffusion measurement is suppression of motion artifacts. Positional changes due to pulsation and respiration of the measurement object may exceed sub-millimeter order and reach several millimeter order, and the positional changes are quite larger compared with ADC. However, since such positional changes due to object motion may be considered coherent phase variations, if DWI can be attained by one measurement, motion artifacts are not particularly generated. In contrast, when phase encoding or signal accumulation is performed, coherent phase variations due to object motion generate motion artifacts. As a method for suppressing motion artifacts in such a case, there has been proposed a method of obtaining navigation data for detecting phase variations due to object motions, and correcting measurement results by using the data (refer to, for example, Patent documents 2 and 3).
There is a measurement called diffusion SI, in which SI and the diffusion measurement are combined, and with separating chemical substances such as metabolites contained in a measurement object, spatial distributions of the molecular diffusion thereof are measured. In the diffusion SI, the aforementioned motion artifacts are generated in a still larger amount. This is because signal accumulation is performed in order to measure metabolites showing lower signal intensity compared with water signals with sufficient accuracy. Moreover, in order to measure ADCs of metabolites showing lower ADC compared with water molecules, it is necessary to perform the diffusion weighting in a greater amount, and therefore a stronger diffusion gradient is added. Therefore, the motion artifacts tend to become larger. In particular, if the phase encoding is performed, which is an imaging method generally used in SI, ghosting artifacts are generated in the direction of the phase encoding. Moreover, the influence of the signal attenuation at the time of signal accumulation cannot be disregarded, either.
There has been proposed a diffusion SI method in which an oscillating gradient is used to decrease the influence of the phase encoding and suppress motion artifacts (refer to, for example, Patent document 4). Furthermore, there has been proposed a diffusion SI method using a method of narrowing a region where the nuclear magnetic resonance phenomenon is induced, which is called line scan, and an oscillating gradient in combination to avoid use of phase encoding (for example, Patent document 5) .